1. Field of the Invention
The present invention relates generally to the restoration of teeth, and more particularly, to methods and devices for improving the accuracy and simplifying the process of performing such restorations by machining a prosthetic, such as a crown or bridge.
2. State of the Art
Presently, numerous methods exist for the restoration of teeth by dentists, including the use of artificial tooth material (such as gold or porcelain) to form a cast-restoration or a metal-ceramic restoration (i.e., dental prosthetics such as crowns). Prosthetic crowns are typically used to repair decayed tooth structure where support from the original tooth structure is either marginal, or unavailable.
Known techniques for preparing a tooth to receive a crown are described in allowed, copending U.S. application Ser. No. 08/785,316, now U.S. Pat. No. 5,813,859, and in published international application PCT/US98/00910, both entitled xe2x80x9cMethod And Apparatus For Tooth Restorationxe2x80x9d, the contents of which are hereby incorporated by reference in their entireties. As described therein, previously known techniques of tooth restoration are susceptible to numerous variables, some of which are within the dentist""s control and some of which are not. All of these variables can detrimentally influence the accuracy with which: (1) the tooth is prepared to receive the crown; (2) the crown is prepared for placement on the tooth; and (3) the manner by which the crown is fit to and fixed on the prepared tooth.
Further, the quality of the prosthetic crown will vary based on the skill of the person who actually produces the crown (e.g., laboratory technician). More particularly, after the patient""s tooth has been shaped to receive the prosthetic crown, an impression is formed from the prepared tooth by placing impression material into the patient""s mouth (i.e., to form a negative impression of the prepared and adjacent teeth). To accurately prepare the impression, all gingival bleeding must be stopped and the margin of the gum tissue must be retracted from the lower portion of the tooth. The impression material must then be properly injected into the sulcus area of the tooth. A tray which contains a combination of impression materials is then applied with pressure over the teeth in the area of the prepared tooth, including the prepared tooth.
Despite efforts by the dentist to obtain an accurate impression of the prepared tooth, many factors can detrimentally influence quality of the impression. For example, the ability of the dentist to maintain a dry field of operation in the area of the prepared tooth can inhibit accuracy of the impression. The retraction of the gingival tissue can also affect the accuracy of the impression, as can the dentist""s technique in obtaining the impression (i.e., the general care in obtaining an accurate impression).
Once the impression has been produced by the dentist, a laboratory technician will set die pins in the impression and then form a master impression as a die (e.g., plaster models) of the patient""s tell. The technician will set the occlusal bite registration and articulate the models of the patient""s teeth. Afterwards, the laboratory technician will saw the die to remove the tooth of interest, then trim the die of the tooth and mark the marginal finish line. The sub-structure is then waxed for preparation of the prosthetic crown.
After a wax pattern has been formed, it is converted (i.e., cast or machined) into a sub-structure (e.g., coping) of the crown. It is a challenge to produce a coping that will comply with acceptable tolerances, given the variables associated with the quality of the impression, the skill of the technician and the proper selection of die materials.
For example, U.S. Pat. No. 5,135,393, assigned to Mikrona, describes a coping mechanism for producing parts such as non-metal copings. As described therein, a three-dimensioned pattern is sensed (e.g., traced) with a feeler pin, and then sensed deflections or displacements of the feeler pin are transferred to a motor driven machining tool. As the pattern is traced, the motor driven machining tool operates upon a blank to fabricate a matching three-dimensional coping. The coping is later used by the dental laboratory to build-up a finished crown.
That is, once the machined coping has been produced, it is processed with a porcelain build-up. The build-up material incorporates specific shading and color effects to simulate the enamel of the original tooth. The porcelain build-up is then vacuum fired.
The combination of producing a coping, followed by building-up the coping with porcelain, are thus required to produce the prosthetic crown. The final stages of crown preparation include finishing the porcelain buildup, after which the anatomy of the original tooth structure is carved therein. The porcelain crown is then glazed. Where the crown is formed of cast metal, the cast exterior of the crown is sand-blasted to remove external oxidation. The metal interior is then polished and the fit, shading and prosthetics of the crown are quality checked. The finished crown is then returned to the dentist for placement onto the prepared tooth structure.
Processes which involve using devices such as those described in U.S. Pat. No. 5,135,393 are not practical for widespread use in dentistry for a variety of reasons. These devices involve complex and timely processes for producing a finished prosthetic suitable for placement in a patient""s mouth.
For example, to produce a finished crown, the process described in the ""393 patent requires: (1) initially making a dental impression of the patient""s teeth; (2) producing a hand made pattern (i.e., template), such as a template of a three-dimensional dental coping from the impression; (3) using an apparatus as described in the ""393 patent to produce a non-metal coping by tracing the template and concurrently machining an oversized blank; (4) building-up the machined, non-metal coping in a dental laboratory with a crown material, such as porcelain; (5) sintering the crown material on the non-metal coping and returning the finished crown to the dentist for final adjustment and placement in the patient""s mouth.
Thus, while an apparatus as described in the ""393 patent is useful in machining dental parts, it does little to reduce the time and complexity associated with producing finished dental prosthetics such as crowns and bridges. The process of shipping an impression from the dentist""s office to the laboratory technician, the preparation of the crown and the returning of the crown to the dentist typically involves a period of approximately two weeks. Upon receipt of the prosthetic crown from the laboratory, the dentist removes a temporary crown which had been placed over the prepared tooth of the patient following preparation of the impression. The permanent crown is then cemented into place. The dentist""s skill is again called upon to ensure proper fit, occlusion bite registration and aesthetics of the prosthetic crown. While the dentist can modify the occlusion of the crown, inaccuracies in fit can require that a new crown be prepared and the entire process described above repeated, thus leading to increased time delays and patient discomfort due to prolonged use of a temporary crown. In some cases, if the crown does not accurately fit, the dentist will use a bur to grind the interior; however, the use of a bur to shape the crown interior alters the fit and therefore detrimentally affects the marginal seal.
The inaccuracies associated with preparation of conventional crowns also affect the preparation and fitting of bridges. For example, where a bridge is formed using a dummy tooth (i.e., a pontic) anchored between two crowns, the inaccuracies in preparing the two crowns will affect the fit of bridge to the prepared teeth of the patient. The difficulties in accurately preparing the pontic will also have an affect on patient comfort. For example, gaps between the pontic and the patient""s ridge structure will allow debris (e.g., food) to be trapped in areas which are difficult to clean.
In the case of root canals, conventional dental prosthetics suffer another disadvantage associated with the use of metal, such as steel posts to anchor the crown. The steel posts are used to reinforce the crown, by anchoring the crown into the tooth structure of the patient""s mouth. However, because the steel posts are typically round in cross-section, they are susceptible to rotation within a post hole drilled in the patient""s bone. As such, the post can loosen, and the crown can fall out of the patient""s mouth. In addition, the post acts as a wedge which is driven into the patient""s tooth structure when pressure is applied to a top surface of the crown. This pressure can cause the root structure of the patient to fracture over time. With regard to root canals performed on visible teeth of the patient, such as the front teeth of the patient, the steel post can produce a visible discoloration of the dental prosthetic, which detracts from the aesthetics of the dental prosthetic.
Thus, it would be desirable to improve the accuracy with which tooth restorations are performed. Further, it would be desirable to reduce the skill-dependent tasks associated with tooth restoration, and to reduce the cost associated with such procedures, without compromising the quality of these procedures. Ideally, it would be desirable to provide a process which would enable a prosthetic to be completely produced in a dental office, within the course of a day, and yet provide a more accurate, aesthetically pleasing dental prosthetic.
The present invention is directed to enhancing the accuracy with which tooth restorations are performed, including the manner by which a tooth is prepared and fit with a dental prosthetic, such as a crown or bridge. Further, the present invention is directed to reducing the skill-dependent tasks associated with tooth restoration, including root canals, while at the same time, improving the precision with which these procedures are performed and the aesthetics of the prosthetic. By improving the accuracy of restoration procedures, any need to repeat these procedures for a given patient can be eliminated and patient comfort can be improved. In addition, by improving the precision with which a prosthetic is prepared for attachment to the prepared tooth of a patient, and/or fit to patient, durability and longevity of the prosthetic are improved.
For example, when the interior of a prosthetic is not precisely fit to the prepared tooth of a patient, as in a case where the coping is undersized relative to the prepared tooth, buckling of the coping can occur. As a result, the buckling of the coping can cause the porcelain exterior of the prosthetic to crack. Because exemplary embodiments of the present invention provide a precise and accurate fit, they avoid such buckling of the prosthetic""s interior, and therefore, improve the longevity of the prosthetic.
Exemplary embodiments of the present invention relate to a method and apparatus for producing a dental prosthetic, such as a dental bridge, wherein a method comprises steps of: providing a prosthetic pontic model; providing a prosthetic pontic blank having exterior dimensions matched to those of said prosthetic pontic model; forming an exterior recess of said prosthetic pontic model as template; and matching an exterior recess of said prosthetic pontic blank to said prosthetic pontic model. Exemplary embodiments in accordance with the present invention can provide a bridge formed with two crowns accurately fit to prepared teeth of a patient, and an accurately prepared pontic which is precisely fit between the two crowns and firmly attached within the patient""s mouth.
Exemplary embodiments of the present invention also relate to a method and associated apparatus for producing a dental prosthetic, such as a crown to be used in connection with a root canal, comprising the steps of: applying light through a light guide to cure a light-curable material; attaching a preformed prosthetic model to said light guide and said light-curable material to form an exterior of said prosthetic model as a template; and matching an exterior of a prosthetic blank to said exterior of said prosthetic model.
Exemplary embodiments of the present invention further relate to an apparatus for producing a dental prosthetic comprising: means for holding a dental prosthetic model and a dental prosthetic blank having exterior dimensions matched to those of said prosthetic model; and means for machining a surface of said dental prosthetic blank to match a surface of said dental prosthetic model, said apparatus providing five axes of motion of said holding means relative to said machining means.